1. Field of the Invention
The present invention relates to a catalyst for a solid polymer fuel cell. More specifically, the present invention relates to a catalyst used for composing a fuel electrode of a solid polymer fuel cell.
2. Description of the Related Art
Fuel cells are highly expected as next-generation power generating systems, among which the use of a solid polymer fuel cell, which employs a solid polymer as an electrolyte, is promising as a power source for electric vehicles, because of a lower working temperature compared with other types of fuel cells, such as a phosphoric acid fuel cell, and of a compact size thereof.
A solid polymer fuel cell has a laminated structure consisting of two electrodes, which are a fuel electrode and an air electrode, and a solid polymer membrane sandwiched by these electrodes; supplies a fuel containing hydrogen to the fuel electrode, and oxygen or air to the air electrode; and takes out an electric power generated in each electrode by an oxidation-reduction reaction. To constitute these electrodes, a mixture of a catalyst for accelerating an electrochemical reaction and a solid polymer is generally used. As the catalyst for constituting the electrode, an electrode catalyst wherein a precious metal having a high catalytic activity is loaded on an electrically conductive material is used, and normally, fine precious metal particles with a size of 2 to 3 nm are loaded on an electrically conductive material, such as carbon, at a ratio (precious metal particles:electrically conductive material) of 90:10 to 50:50, is used.
Various properties are required to a catalyst for an electrode of a solid polymer fuel cell. To the catalyst for a fuel cell, although poisoning resistance against carbon monoxide in the fuel has been required, new problems have arisen in these years when the practical use of solid polymer fuel cells has been established. An example of these problems is a deterioration of cell properties due to fuel starvation, which can occur during the operation of a fuel cell. In the new problem, when an abnormality is caused in the fuel supply for some reasons during the normal operation of the fuel cell, the catalytic activity in the fuel cell degrades due to fuel starvation, causing the deterioration of cell properties, to thereby bringing about obstacles to the routine power supply. It has been confirmed that such lowering of catalytic activity due to fuel starvation is irreversible, and even if the fuel is supplied again, the catalytic activity cannot be completely restored.
Here, it can be said that a countermeasure to address the problem of irreversible inactivation of the catalyst is to establish the system wherein fuel supply is not stopped. However, even if the improvement of such a peripheral system is feasible, it is preferable to drastically improve the fuel electrode and catalyst considering in case of emergency.
Related to the problem of fuel starvation, as a countermeasure for a fuel cell catalyst heretofore studied, for example, the addition of ruthenium oxide (RuO2) or iridium oxide (IrO2) to the catalyst layer has been known. As another improving method, it has been known to be effective to use a support that is stable to oxidation properties, such as graphite carbon and titanium oxide (Ti4O7), and to increase the loading of catalyst particles (refer to International Patent Laid-Open Nos. WO01/15247 and WO01/15254).
The present inventors have proposed a method for solving the problem of fuel starvation in view of the improvement of an electrode structure. The electrode has a multilayer structure composed of a reaction layer that proceeds the fuel cell reaction, and a water decomposing layer that electrolyzes water in the fuel electrode (refer to Japanese Patent Application Laid-Open No. 2004-22503).
According to the examinations by the present inventors, a prima-facie effect can be seen in the above-described various countermeasures. However, the former improvement of the catalyst is not necessarily sufficient, and indispensable deterioration in properties is seen in fuel starvation. Although the above-described improvement of the electrode is effective, it can be said that the catalyst itself is improved to make the technique more effective.
The present invention has been made in the above-described background, and an object of the present invention is to provide a catalyst for a fuel electrode of a solid polymer fuel cell that has been more effectively improved in view of fuel starvation, and is difficult to cause the deterioration of catalyst activity even if fuel starvation occurs.